1. Field of the Invention
The present invention relates to a transmission line for handling high frequency signals in a microwave band, a millimeter wave band and the like and a semiconductor integrated circuit device having the transmission line.
2. Description of the Related Art
In a conventional communication apparatus using high frequency signals in a microwave band, a millimeter wave band, and the like as carrier waves, a transmission line such as a microstrip and a coplanar waveguide has generally been used as a bias supplying circuit for supplying power to an active device.
FIGS. 22A and 22B are schematic sectional views respectively showing a structure of an ordinary microstrip and a structure of an ordinary coplanar waveguide.
As shown in FIG. 22A, the microstrip has a dielectric substrate 101, a signal strip 102 disposed on a top face of the dielectric substrate 101, a ground conductor layer 103 disposed on a bottom face of the dielectric substrate 101 as opposed to the signal strip 102 with the dielectric substrate 101 disposed between the ground conductor layer 103 and the signal strip 102.
As shown in FIG. 22B, the coplanar waveguide has a dielectric substrate 101, a signal strip 102 disposed on a top face of the dielectric substrate 101, a pair of ground conductor layers 104 disposed on a bottom face of the dielectric substrate 101 in such a manner as to face the signal strip 102 with a predetermined spacing in the width direction of the signal strip 102.
To a main signal circuit of the communication apparatus, an arbitrary number of bias terminals for supplying a common voltage to the main signal circuit are electrically connected through the bias supplying circuit having the transmission line shown in FIG. 22A or FIG. 22B. The communication apparatus is typically composed of a microwave monolithic integrated circuit (hereinafter abbreviated as “MMIC”) that is a semiconductor integrated circuit wherein a transmission line, an active element, a passive element and the like are provided on its common dielectric substrate and peripheral circuits accompanying the MMIC.
In general, in a module used as the communication apparatus, it is necessary to transmit the carrier waves efficiently. Accordingly, in regions of the MMIC and the peripheral circuits where the carrier waves are transmitted, it is necessary that the dielectric substrate constituting the circuits is formed from a low loss material and the signal strip is formed from a high conductivity (low resistance) material.
In a known MMIC, gallium arsenide which is the low loss material is used as a dielectric substrate material, and a transmission line, an active element, a passive element, and the like are disposed on a common dielectric substrate made of such material.
FIG. 23 is a circuit diagram showing a circuit structure at the output side of a module functioning as a high frequency amplifier that is a first prior art. In the module shown in the figure, the MMIC is provided with a main signal circuit 110 having an active element 111, an output terminal Tout, main signal lines 112a and 112b for electrically connecting the active element 111 and the output terminal Tout to each other, and a DC blocking capacitor 118. In the main signal circuit 110 of the MMIC thus-constituted, an input signal received by an input unit (not shown) is amplified by the active element 111 and then an output signal from the active element is outputted from the output terminal Tout through the main signal lines 112a and 112b. The MMIC is further provided with a short stub 113 branching from a portion between the main signal lines 112a and 112b and a first bypass condenser 114 disposed between the short stub 113 and a ground conductor. Further, the module itself is provided with a bias supplying circuit 120A for supplying a power voltage to the MMIC, and the bias supplying circuit 120A is provided with a bias terminal Tvd for supplying a DC power voltage, transmission lines 115 and 116 connected serially, and a second bypass condenser 117 disposed between a node of the transmission lines 115 and 116 and the ground conductor.
Here, the short stub 113 functions as a part of the bias supplying circuit 120A as well as a matching circuit for the main signal circuit 110 in the RF (Radio Frequency) band. A capacitance value C1 of the first bypass condenser 114 is set to such a value that a high frequency signal included in the design frequency band is short-circuited. A capacitance value C2 of the second bypass condenser 117 is set to such a large value at which a high frequency signal included in a low frequency band is short-circuited, the second bypass condenser 117 being an external type chip condenser in this prior art.
In general, in the communication apparatus, the high frequency signal may leak to the bias supplying circuit 120A if the high frequency signal is not short-circuited in the bias supplying passage (bias supplying circuit 120A) from the main signal circuit 110 to the bias terminal Tvd. For example, a parasitic oscillation may occur in a multistage amplifier in the case where connection of the transmission line constituting the bias supplying circuit is in such a fashion that it causes a positive feed back from a rear stage amplifier to a front stage amplifier. Therefore, in the module shown in FIG. 23, the bypass condensers 114 and 117 are provided between the ground conductor and both ends of the transmission line 115 which is a part of the transmission line constituting the bias supplying line in such an arrangement as to achieve shunting, thereby short-circuiting high frequency signals of various frequency components that can be amplified by the active element.
However, many problems are left unsolved with the conventional transmission lines and the communication apparatuses having the transmission lines.
For example, in the module (amplifier) shown in FIG. 23, conditions for sufficiently short-circuiting the high frequency signals of various frequency components that can be amplified by the active element 111 are not satisfied in the bias supplying passage from the main signal circuit 110 to the bias terminal Tvd. Therefore, there has been a problem that high frequency isolation characteristics between the elements and between the terminals both connected by way of the transmission line are not satisfactory. More specifically, a high capacitance chip condenser (for example, the second bypass condenser 117 shown in FIG. 23) designed for short-circuiting a low frequency band of a several tens of megahertz has a difficulty in short-circuiting a high frequency band of about a several gigahertz or more because the chip condenser has a parasitic component such as grounded capacitance. Thus, in an amplifying element structure serially connected in a general multistage wherein a rear stage active element and a front stage active element are connected to an identical bias supplying circuit, the parasitic oscillation due to the positive feedback may occur. The parasitic oscillation occurs when a high frequency signal is amplified by the rear stage active element and a component of the high frequency signal that leaks out to the bias supplying circuit of the output side and is not short-circuited is input to the front stage active element through the bias supplying circuit under the phase condition of the positive feedback.
Also, a resonance may occur due to capacitance of the first bypass condenser 114 and inductance of the transmission lines 115 and 116 of the bias supplying circuit. In this case, since a standing wave is generated to cause radiation in the transmission line 115, an unintentional connection may occur between the transmission line 115 and the peripheral circuits in a resonance frequency. Further, a transmission characteristic of the signal in the main signal circuit 110 that is connected to the short stub 113 is unintentionally improved in the resonance frequency. Consequently, a peak of unnecessary gain is generated in the resonance frequency as a characteristic of the overall amplifier.
FIG. 24 is a circuit diagram showing a circuit structure at the output side of a high frequency amplifier (module) of a second prior art in which a structure for reducing Q value of the resonance is supplemented. As shown in FIG. 24, this MMIC has a structure wherein instability is improved through attenuation of the low frequency component by disposing a resister 119 having a resistance value of R1 between the transmission line 115a and the transmission line 115b of the bias supplying circuit 120B.
However, in the structure of FIG. 24, it is necessary to set the electric resistance of the resister 119 to a large value for the purpose of eliminating the low frequency component, and, with such large electric resistance, a voltage drop of the power voltage supplied from the bias terminal Tvd is large. That is to say, a reduction in driving voltage of the MMIC may entail a drawback of deteriorating an amplifying efficiency in the MMIC and the like.
FIG. 25 is a block circuit diagram showing a circuit structure at the output side of a high frequency amplifier (module) of a third prior art in which a structure for reducing Q value of the resonance is supplemented. This high frequency amplifier is disclosed in the literature of Cheng et al.: One Watt Q-Band Class A Pseudomorphic HEMT MMIC Amplifier, 1994, IEEE MTT-S Digest, p.p. 805-808. To this circuit structure example, a method of short-circuiting a bias supplying circuit 120C by an RC serial circuit 123 in parallel with the bias supplying circuit 120C is adapted. The output circuit of the high frequency amplifier of FIG. 25 is different from that of the high frequency circuit of FIG. 23 in that the transmission line 115 to which shunt capacitances (the first bypass condenser 114 and the second bypass condenser 117) are connected at its ends in the output circuit of the high frequency amplifier of FIG. 23 is divided into transmission lines 115a and 115b and that a third bypass condenser 122 is additionally connected to a node of the transmission lines 115a and 115b to achieve the shunt arrangement. Further, a resister 121 having a resistance value of R2 is disposed between the node of the transmission lines 115a and 115b and the third bypass condenser 122. In other words, the RC serial circuit 123 functioning as a stabilizing circuit is provided between a part of the bias supplying circuit 120C and the ground conductor in the output circuit of the high frequency amplifier of FIG. 25.
A capacitance value C3 of a third bypass condenser 122 is so set as to short-circuit a high frequency signal of an intermediate frequency band that is not short-circuited by the first and the second condensers 114 and 117. The resister 121 is provided so as to reduce the unnecessary gain in the high frequency signal of a low frequency band lower than the design frequency band and to cause loss to be generated in the high frequency signal of the intermediate frequency band and short-circuit it for the purpose of improving stability of the high frequency amplifier.
However, in the high frequency amplifier shown in FIG. 25, it is necessary to provide additionally the bypass condenser 122 having a capacitance value sufficient for short-circuiting the high frequency signal of intermediate frequency and the resister 121 in the high frequency amplifier shown in FIG. 23, thereby undesirably increasing a circuit area in the whole module.
Also, it is necessary to add a via hole as a ground circuit in the high frequency amplifier using the microstrip as the transmission line, and such additional component is not preferred as it further increases the circuit area.
In the high frequency amplifier shown in FIG. 25, if the RC serial circuit 123 is disposed in the vicinity of another circuit element, electromagnetic coupling with another circuit (e.g. the main signal circuit 110) occurs to cause the drawback of making the high frequency amplifier instable. The RC serial circuit 123 could be disposed remote from the main signal circuit in order to avoid such electromagnetic coupling, but such arrangement is not preferred since it further increases the circuit area.
The above described drawbacks exist in the semiconductor integrated circuit device other than the amplifier, such as a mixer, a frequency multiplier, a switch, an attenuator, a frequency demultiplier, and an orthogonal modulator.